Aneurysms are blood-filled dilations of a blood vessel generally caused by disease or weakening of the blood vessel wall. The wall of the aneurysm may progressively thin, which increases the risk of rupture causing hemorrhagic stroke or even sudden death. There are about 30,000 to 40,000 cases of aneurysmal rupture per year in the United States, accounting for about 5% of all strokes. The prognosis after aneurysmal rupture is poor; the 30-day mortality rate is approximately 45% and a positive functional outcome is achieved in only 40-50% of survivors. Traditional approaches to preventing aneurysmal rupture often include packing the aneurysm with metal coils to reduce the inflow of blood to the aneurysm and prevent further enlargement and rupture. Such coils are often referred to as “embolic coils” or “microcoils,” and can be categorized into the following three groups based on their structural properties: framing coils, filling coils, and finishing coils. Framing coils are inserted first into the aneurysm and form the base structure into which the later-delivered filling coils are packed. As such, framing coils are stiffer than filling and finishing coils to provide structural stability and generally have a complex or three-dimensional shape for approximating the periphery of the aneurysm. Filling coils, in contrast, are softer than framing coils, and multiple filling coils are packed within the framework of the framing coil(s) to achieve a high packing density. Finishing coils are delivered last to fill any remaining gaps left between filling coils.
Embolic coils, however, have several drawbacks. First, embolic coils generally only achieve a 20-40% packing density (i.e., ratio of the volume of the coils inserted into the aneurysm sac and the volume of the aneurysm sac). As a result, blood continues to flow into the aneurysm (also known as recanalization) in about 30% of coil cases, which can cause further swelling of the aneurysm over time. In addition, because the coils must be very small to fit within a microcatheter for delivery through the tiny cranial vessels, numerous coils are often required to adequately fill the aneurysm. These numerous coils must be delivered one-by-one, thereby increasing procedure time and complexity. Yet another drawback is that embolic coils cannot accommodate the wide range of aneurysm shapes and sizes. Embolic coils, for example, are difficult to stabilize within wide-necked aneurysms, which can result in migration of one or more coils across the neck such that a portion of the migrated coil(s) protrudes into the parent blood vessel. The protruding portion of the migrated coil(s) can be a nidus for thromboembolism, which can be fatal if left unaddressed. To address this shortcoming, many existing treatments include positioning an intracranial stent across the neck of the aneurysm to prevent all or part of a coil from migrating across the neck. However, intracranial stents can also be a nidus for thromboembolism, and further increase procedure time and cost. Thus, there is a need for improved devices, systems, and methods for treating aneurysms.